Have to agree that Cory's are an all round great fish, provided they are given the correct care and not expected to just "clean up" after other fish. I have a 4ft tank dedicated to just cory's and shrimp (now also with a collection of Borneo Suckers) and IMO I could not have a more rewarding tank in terms of hardiness, ease of care, and peacefulness. Guppies I find are hardy (well ours seem to be) but frustrating in terms of population explosions just as platty's and swordtails can be. Neon and cardinal tetras seem to be rather finiky and drop dead in a blink of an eye, while my black neon tetras have never looked backwards and are powering along in their 4ft tank with more shrimp and Bristlenose catfish. Khuli loaches too I have found to be wonderful additions to any tank and never a bother in regards to other fish kept with them.
So really fish to avoid comes down to tank size, owners experience with other fish species and water condions for the area, and most importantly what sort of tank you want to have. Do you want a solitary predatory fish, a brackish tank with semi aggressive fish or a peaceful placid tropical tank, or even slightly pugnacious chiclids the possibilities are seemingly endless but each situation will have fish that are considered to be easy or hard to keep and maintain.